What does it mean?
U1187 is a Network (CAN bus, module communications) fault code. It indicates: Manufacturer-Specific CAN/Network Bus Fault . This is a standardised OBD-II code. The technical definition is the same regardless of the make or model of vehicle, although specific causes and symptoms can vary between vehicles.
Symptoms
- Several warning lamps illuminated at once with no single obvious mechanical fault
- Modules missing or intermittently disappearing from the scan tool list
- Functions dropping out and coming back on their own while driving
- The code returning quickly after being cleared, often under vibration or in damp weather
- Slow wake-up, delayed gauge response or an unexplained battery drain
Likely causes
- Damaged bus wiring, most often at a flex point, a grommet or where the harness rubs metal
- Water and corrosion in a shared connector, splice or ground point
- A module with a failing transceiver that disturbs communication for every other unit on the branch
- Low battery voltage, a bad main ground or charging system noise causing modules to reset
- Termination resistance out of specification after a module was removed or a harness modified
- Aftermarket electronics connected to the data lines
- Software or coding left inconsistent after a module replacement
How to fix it
- Scan every module, list the ones that answer, and map which network branch the silent units share
- Check the battery, charging voltage and main ground connections before any harness work
- With power removed, measure resistance across the CAN pair to confirm both terminating resistors
- Back-probe the bus lines and compare them against a healthy branch, then wiggle test the harness at flex points and grommets
- Inspect shared connectors, splices and ground straps for corrosion, water and pin damage
- Disconnect modules one at a time until the network recovers, which isolates the failing unit
- Repair the wiring, verify coding and software, and replace a module only once it is proven to be the fault